Modern high power x-ray tubes for CV Cardio Vascular and CT (computer tomography) applications are rotating anode tubes. Here an accelerated electron beam hits a rotating anode disc, which generates x-ray photons. Typically the electrons are emitted from a thermal emitter, i.e. the total emission current and thus the total x-ray power depends on the emitter temperature. For a given acceleration voltage, which is given by the required type of examination, the x-ray output and thus the x-ray dose given to the patient can only be changed by changing the emitter temperature. This is done in modern CT-scanners, where the dose is adjusted according to the elliptical shape of the patient cross section. Unfortunately only a relatively small modulation depth can be obtained, since the thermal cycling of the emitter is slow due to its large heat capacity. Another method to implement dose modulation for CT would be to operate the tube in a pulsed mode and adjust the dose via pulse width modulation.
X-ray tubes for CT systems normally operate in a continuous mode, while tubes for CV applications can also work in a pulsed mode, where the pulse width is in the order of a few milliseconds. The required on- and off-switching of the electron beam cannot be done by changing the emitter temperature accordingly, since this process is far too slow. Instead an additional grid electrode is introduced close to the emitter. The beam can then be switched off by applying a sufficiently large negative voltage to the grid electrode with respect to the emitter. The required voltage is normally in the range of several kilovolts. In principle, such a grid electrode could also be used within a CT x-ray tube to enable a pulse mode operation for dose modulation. Unfortunately the required pulse widths are then very short (10-100 microseconds) and high repetition rates of up to 20 kHz would be required. This cannot be realized with state of the art grid switch drivers and typical capacitive loads of a few hundred picofarad, arising from the grid electrode and the required high voltage cable.
The typical solution for driving a high-voltage capacitive load is to directly drive it with a switch capable of handling the high peak currents (=hard-switching). In this case the capacitive energy E=(C*V^2)/2 is simply dissipated inside the driving switch. The maximum operating frequency of this solution is determined by thermal limitations of the switch, and also the power supply needs to deliver this power, which is lost afterwards.
Another known way of driving a high-voltage capacitive load is by using flyback power supplies, which generates fewer losses than the hard-switching topology, but have the drawback of being less fast and taking much space.
From “Advance Soft-Switching Sinewave PWM High-Frequency Inverter-Link Cycloconverter Incorporating Voltage-Clamped Quasi-Resonant and Capacitive Snubber Techniques” of H. Yonemori and M. Nakaoka, Conference Record of the 1991 IEEE Industry Applications Society Annual Meeting in Dearborn, Mich., USA, 28 Sep.-4 Oct. 1991, power conversion circuits are known with a high-frequency AC link.